Rolling cutter bits of a conventional three-cone design are extensively used in the drilling of bore holes for hydrocarbon production. The usual design of a rolling cutter bit consists of three cone shaped cutters each journaled to rotate about a journal pin. Each of the journal pins is equally spaced, 120 degrees apart, with the three pins mounted to a bit body. The entire structure is rotated at the end of a drill string. While an asymmetrical three cone rock bit with unequal sized cones is known, for example, see U.S. Pat. No. 3,397,751, for the more usual case, all three cones of a bit are the same size and have the same base diameter.
A reoccurring problem of the drilling industry is the deviated or crocked bore hole that results when the drill bit encounters formations that are not horizontal. Ideally, the bore hole should be vertical or substantially vertical. However, the encountering of non-horizontal formations often results in a deviation from the vertical. Undesirable deviation has also been known to result when the drill bit strikes a buried obstacle or by the passage of the drill bit from one strata to another strata having a different degree of density. A deviated bore hole not only increases the cost of drilling, but also has been known to cause the bore hole to bypass the productive sands.
Some of the early drill bits used for drilling bore holes for hydrocarbon production consisted of two rotary cutter cones each journaled to rotate on a journal pin, where the journal pins were equidistant from each other circumferentially. Although it is recognized that the three cone rotary rock bit is the most efficient arrangement under most drilling conditions, it has been found that the two-cone bit will tend to bring a deviated bore hole back to a vertical alignment more effectively than a three-cone bit. The two-cone bit has been found to take advantage of the pendulum effect which is the natural tendency of a bit suspended from a drilling rig at the earth's surface by a drill string to seek a vertical position. Thus, the two-cone bit provides a more efficient device for correcting a deviated bore hole. However, the conventional two-cone bit has the tendency to set up drill string vibrations with destructive effects on the surface equipment.
The three cone rotary rock bit is recognized as having many advantages, including the minimization of vibration of the drill string. In accordance with the present invention there is provided an asymmetrical rotary cone bit that has the advantages of a two-cone bit to minimize deviation or correct bore hole deviation, and the advantages of a three-cone bit including the minimization of drill string vibration. Previous attempts to provide the advantages of a two-cone bit and advantages of the three-cone bit are known in the prior art, for example, see U.S. Pat. No. 3,142,347.
For each revolution of the two-cone bit there is one less cone in contact with the wall of the bore hole which makes the bit more responsive to the "pendulum effect." While this pendulum effect has the tendency of maintaining a vertical bore hole, the resistance to gage wear in the two-cone bit is reduced to two-thirds that of the conventional three-cone rotary drill bit. As mentioned, the two-cone bit will run much rougher than a three-cone rotary drill bit with excessive vibrations to the upstream equipment. However, the two-cone bit is more effective because by enabling the use of larger cones, less weight is needed for penetration of the rock strata.
In accordance with the present invention, there is provided an asymmetrical rotary cone bit that utilizes the larger cones of the two-cone bit but provides a third cone to achieve the advantages of the three-cone bit. Thus the asymmetrical cone bit of the present invention approaches the two-cone structure with two large cones for the pendulum effect, while retaining the advantages of the three-cone rotary bit to hold the gage dimension.
A further advantage of the two-cone bit is that it enables the use of substantially larger bearings with correspondingly longer bit life and further increases the number of cutting teeth per cone which also increases bit life. It is well recognized in the drilling industry that any increase in bit life substantially reduces overall drilling costs by reducing the number of "trips" of the drill string from the bore hole to change bits.